Nitrification is the first step for the removal of nitrogen from wastewater, where ammonium (NH4+) is oxidized to nitrate (NO3−) by aerobic, autotrophic, nitrifying bacteria. These bacteria are thought to have slow growth rates and are sensitive to pH and temperature swings, making nitrification difficult to maintain in activated sludge systems (Mobarry et al. 1996; Wagner et al. 1996). The slow growth rate and associated nitrification rate requires a lengthy solids retention time (SRT), as much as 20 days. Previous work has demonstrated that the growth of some autotrophic bacteria is carbon limited (Dagley and Hinshelwood 1938; Green et al. 2002; Denecke and Liebig 2003). Inorganic carbon was found to be a limiting factor in biological nutrient removal (BNR) systems due to the low partial pressure of carbon dioxide (pCO2) of the atmospheric air introduced, and the loss of CO2 by stripping (Wett and Rauch 2003). These factors were reported to limit the bulk concentration of CO2 in wastewater and consequently affect nitrification.
The art lacks, however, an evaluation of the effect of elevated pCO2 on the specific growth rate of nitrifying bacteria using activated sludge from three different types of BNR processes: extended-aeration, Modified Ludzack-Ettinger (MLE), and Bardenpho.